Conventionally, as shown in FIG. 3 (a), comprising a large end 11 and a small end 12 used in a synchro-mesh mechanism, a reverse-tapered tooth part 12a is formed on the outer circumference of the small end 12 to compose a gear 1A for a speed change device, separate large-end gear and small-end gear are integrated by electron beam welding or spline fitting means, and an integral structure is formed by forging or the like (see, for example, patent document 1).
In particular, the gears integrated by electron beam welding require an assembling process, and adverse effects due to welding strain may be considered, or in the case of the gears integrated by spline fitting means, an assembling process is needed, and splines must be formed, and the spline forming process is complicated, and generally the gears are integrally formed by forging.
On the other hand, from the viewpoint of downsizing and compact design of an automobile, as shown in FIGS. 3 (b) and (c), for reduction of thickness, a small end 12 forming a reverse-tapered gear part on the outer circumference is provided in a rim 11a of a large end, and gears 1B, 1C for a speed change device of so-called sink type are widely employed.
Incidentally, since the reverse-tapered tooth part 12a is formed by a die which projects horizontally from the outside of the radiation direction toward the center, out of such sink type gears 1B, 1C, as shown in FIG. 3 (b), in the case of the gear 1B smaller in the degree of sink, and larger in the groove width of an annular groove 11b formed between the reverse-tapered tooth part 12a and the rim 11a, by using a hook-shaped die bent so as to avoid the rim 11a, it is possible to form integrally by forging, but as shown in FIG. 3 (c), in the case of the gear 1C smaller in the groove width of an annular groove 11b formed between the reverse-tapered tooth part 12a and the rim 11a, and larger in the degree of sink in order to reduce the thickness, it was impossible to form by forging.
To solve this problem, a new manufacturing method of gears for a speed change device is proposed, in which a large end and a small end are overlapped in a stepped doughnut shape, a reverse-tapered tooth part is formed on the outer circumference of the small end to manufacture a primary formed part, and a range excluding the small end including the reverse-tapered tooth part, and the rim forming portion at the large end continuous to the small end is confined, and the peripheral wall thickness at the large end is extruded to the small end side, and by this extruded wall thickness portion, a rim surrounding the small end concentrically is formed (see, for example, patent document 2), but since a large load is generated when extruding the peripheral wall thickness at the large end toward the small end side, the manufacturing apparatus becomes larger in size, and deformation of gears is deviated locally, and strains are likely to occur in the gears.